The present invention relates to methods and devices for the measurement of physical parameters using a class of optical probes which permit the measurement of temperature by at least two physically independent optical methods, or the measurement of temperature in addition to at least another physical parameter, using a single optical probe, a single interrogating light source and a single photodetector, in conjunction with fiber optic techniques.
Prior art methods and devices for measuring temperature include the use of probes made of materials having temperature-dependent photo-luminescence properties. One of the earliest systems was described in U.S. Pat. No. 2,551,650 of Urbach, and used a photo-luminescent material the luminescence intensity of which was quenched appreciably with an increase of temperature. Luminescence quenching is usually associated with a decrease of the luminescence decay time of the material following excitation of its luminescence by pulsed or oscillatory light of wavelengths within an electronic absorption band characteristic of the material. Since the measurement of a luminescence decay time is usually more accurate and reliable than the measurement of a luminescence intensity (especially in the absence of intensity referencing), some recent temperature measurement techniques using photo-luminescent probes have used the temperature-dependent luminescence decay time as temperature indicator. These decay time techniques were used in a plurality of fiber optic temperature measuring techniques, including among others those described in U.S. Pat. Nos. 4,223,226 and 4,245,507 and in a publication by J. S. McCormack (Electronics Letters 17, 630 [1981]). These prior art techniques have, however, a serious disadvantage: As temperature increases, the signal strength and, hence, the measurement accuracy, decrease. This limits severely the temperature range of operation of probes which have a temperature coefficient of decay time of the order of one percent or better, so a wide temperature range can be achieved only with probes having a significantly lower decay time coefficient and, hence, a significantly lower sensitivity and accuracy.
In many situations encountered in industry, it is necessary to measure a physical parameter, for example pressure, under temperature-varying conditions. Temperature variations may affect the performance of the pressure sensor, so it is often necessary to measure the temperature of the pressure sensor in order to introduce the appropriate correction factors for processing the pressure readings. This usually requires a temperature measuring device in addition to the pressure sensor, which increases the complexity of the instrument.